A number of methods have been investigated for reducing the size of vias between metal layers. Current conventional techniques include the pillar method and the chemical vapor deposition - tungsten via fill method.
The pillar process provides a metal plug connecting two layers of metal, which plug may be self-aligned on the edge of the lower level metal. The pillar is nonnested, i.e., does not require that the patterning be inside the lower level lead lateral boundary. The pillar process includes an oxide planarization which provides a flat surface for the upper level metal. This process has, however, a high process complexity, an additional photomask level when compared to a standard via process, and a very critical planarization process that is susceptible to interlevel shorts.
The conventional via fill process allows vias to be etched with straight walls that greatly improve size control when compared to sloped vias. However, these vias must be aligned to the lower metal layer, and therefore, are required to be nested within the leads. For the conventional via fill process, therefore, it is necessary to either flare the leads or further reduce the via size. The former technique reduces the metal packing density, and the latter technique increases via resistance and current density.
A need has therefore arisen in the industry for a method for building a via which is self-aligned to the edge of the lower metal layer and takes advantage of the CVD-tungsten via fill method. A need has further appeared in the industry for a process that greatly reduces the likelihood of interlevel shorts.